Computer-to-plate (CTP) imaging devices use laser diodes for imaging on a substrate. Leakage of laser light from the imaging device during imaging operation may be hazardous to people in proximity to the device. Various safety measures may be required as high power invisible laser (IR) beams are present at least in the optical fibers of the device during operation and could cause eye damage if they leaked from the device. Interlock sensors are deployed in the device at various locations to notify of hazardous situations or to prevent, for example, opening the device door during operation. The sensors are integrated with specific electronic circuitry as shown in FIG. 1. When one or more of the interlock and safety sensors goes off, indicating a potential leak of laser light, the laser sources are disabled and fault warning activated.
Safety devices are mandatory in potentially hazardous products, and are required to be installed by international safety standards. Safety measures in devices can be incorporated by various means such as software, electronic CPLD components or electro-mechanical components. In the case of CPLD components, the CPLD components are installed on an circuit board, which may include more than one CPLD component. One of the CPLD components may be used for safety.
CPLD components are programmable and the programming is done via a joint test action group (JTAG) port residing on the electronic board and connected typically to a personal computer by an appropriate cable. The CPLD reprogramming may be done by service engineers in the field, therefore it is essential to prevent reprogramming of the CPLD components related to safety and make sure they are programmed only at the manufacturing site. Reprogramming of CPLDs which are intended for safety control in the field is dangerous since it may hinder safe operation of the product, and thus may cause irreversible harm to the product operators, during product operation.
FIG. 1 shows several CPLD components (124, 128, 132) residing on electronic board 100. The JTAG interface is connected to each of the CPLD components by input and output pins (TDI, TDO) and timing and clock pins (TMS, TCK). The signals TDI, TDO, TMS and TCK are used for accessing and programming of the CPLDs residing on electronic board 100. The CPLD components are concatenated between each other via the input and output pins (TDI, TDO). The TDI signal from JTAG interface 108 is connected to first CPLD 124 via its TDI line. The output line TDO of CPLD 124 is further connected to the input line TDI of the next CPLD in the chain 128. The output pin TDO of the last CPLD in the chain 132 is connected to the TDO line of JTAG interface 108 to form a complete chain of the CPLDs on electronic board 100. The timing TMS and clock pin TCK are connected in parallel from JTAG interface 108 to all CPLDs (124, 128, 132) and controlled by timing and clock pins (TMS, TCK).
The programming of the CPLD components is performed by an external controller 104. The programming data is delivered to JTAG interface 108 via interface lines 112. The content of each of the CPLD components (124, 128, 132) can be independently programmed by controller 104. CPLD 124, which is selected to serve safety measures, has to be treated with more caution than other CPLDs, therefore programming of CPLD 124 should be permitted only once and at the manufacturing site only. Programming at a customer site is unacceptable.